Conventional angioplasty balloons are typically constructed of low-compliance materials that tolerate high inflation pressures and attain uniform predictable diameters in vivo even when portions of the surrounding artery are narrow and calcified. The typical balloon has a cylindrical section of uniform diameter between conical ends and a central catheter extending along the longitudinal axis of the balloon. When inflated at high-pressures, the walls of the balloon are placed into tension and the balloon becomes stiff and biased into a straightened configuration. Such balloons impose this straightened cylindrical configuration on any balloon-expanded stent that is crimped or otherwise loaded upon the balloon for expansion.
The presence of a straight stent in a curved artery (e.g., coronary, renal, femoral arteries, and the like) imparts stresses and strains into the stent structure, the artery, or both, especially when the artery is mobile. The resulting repetitive micro-trauma may incite inflammation, hyperplasia and recurrent narrowing, often to the point of catastrophic flow limitation.
Previous efforts to imbue an angioplasty balloon with flexibility have employed segmentation, helical shape, and compliant balloon materials. Segmented balloons take a variety of forms depending on the degree of segmentation. For instance, previous devices have included spherical balloons strung out along a central catheter having narrow intervening segments that are easily bent. However, such balloons are ill-suited to stent delivery because they impose a bumpy segmented shape upon the stent. If a segmented balloon is inflated enough to eliminate inter-segment gaps and deliver a more completely expanded stent, adjacent segments interfere with one another hindering much of the flexibility.
Other suggested balloons have included adjacent segments that are separated by grooves in an otherwise continuous balloon. These localized “hinge-points” do little to enhance differential lengthening and the effect on balloon flexibility is therefore modest at best. Other balloons have deep grooves that separate bulges in the balloon profile but these too have a modest effect on flexibility.
Helical balloons have also been used to increase flexibility where the winding of the balloon disrupts longitudinal continuity so that adjacent windings on the outer aspect of a bend in the balloon can separate while those on the inner aspect remain in close apposition. The resulting potential for differential lengthening imparts some flexibility. In addition, helical balloons benefit from multi-lumen construction. Each of the component balloons is narrower and therefore more flexible than the resulting helix. However, such helical balloons suffer many of the same limitations as segmented balloons. They either deliver incompletely expanded stents or become less flexible when overinflated to eliminate gaps. Moreover, even when the balloon is straight, its components have tight bends that, unless tightly constrained, straighten on high-pressure inflation, whereupon the helical balloon may tear itself apart. A non-compliant, tightly-wound, helical balloon may potentially tear itself apart upon high-pressure inflation. A less tightly wound helical balloon is more stable but less flexible.
Balloons that are constructed from compliant materials are more flexible than similar balloons constructed of non-compliant materials. However, compliant balloons cannot withstand the high pressures required for balloon angioplasty because they tend to expand in the direction of least resistance, leaving narrow areas untreated, rupturing the artery in areas of weakness, and/or spreading beyond the intended field of angioplasty. In addition, compliant balloons may be unable to generate sufficient force to initiate stent expansion. Early angioplasty balloons made of compliant materials were subsequently reinforced by the application of various braids, meshes, and wraps in an attempt to control balloon shape and dimensions at higher working pressures.
External braids, wraps, and fabrics of all kinds have also been embedded into the walls of low-compliance balloons to further increase the maximum working pressure. However, the integration of a braid into the low-compliance wall of a high-pressure balloon prevents changes in braid angle. The braid of such a balloon is not free to open and close, or shorten and lengthen, with balloon expansion and contraction. Consequently, the presence of the braid does nothing to shorten the balloon, relieve longitudinally-directed tension, generate redundant folds in its walls, enhance flexibility, or prevent forcible straightening during inflation.
Accordingly, there exists a need for balloons that, when inflated to high-pressure, remain flexible and/or retain a curved configuration.